1. Technical Field
The present invention relates to a media stripper mechanism, a fixing device, and an image forming apparatus, and more particularly, to a media stripper mechanism for stripping a recording medium from a rotary member, which can be employed in a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine.
2. Background Art
In electrophotographic image forming apparatuses, such as printers and photocopiers, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and settling the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image onto the medium under heat and pressure.
One such fixing device includes a multi-roller, belt-based fuser assembly that employs an endless, flexible fuser belt entrained around multiple rollers, one of which is equipped with an internal heater to heat the length of the fuser belt through contact with the heated roller. Owing to the fuser belt which exhibits a relatively low heat capacity and therefore can be swiftly heated, the belt-based fuser assembly allows for shorter start-up time and smaller amounts of energy wasted during standby, as well as a relatively compact size of the fuser assembly.
One important factor that determines imaging quality of a fixing device is the ability to properly convey a recording medium through the fixing nip without causing the recording medium to wind or wrap around the rotary fixing member. Media wraparound occurs where the toner image heated through the fixing nip becomes sticky and thus adheres to the surface of the fixing member upon exiting the fixing nip. If not corrected, a recording medium wrapping around the fixing member would cause jam or other conveyance failure in the fixing nip.
For obtaining a fixing process with high immunity against media wraparound and concomitant conveyance failure, a fixing device may use a fuser roller or belt coated with a release agent such as fluorine resin where it contacts a heated, sticky toner image in the fixing nip, while equipped with a media stripping member in the shape of a claw which is held in contact with the fuser member to separate a recording medium from the fuser member at the exit of the fixing nip. Use of the media stripping claw, however, can cause image defects depending on a specific application of the fixing device.
For example, for monochrome printing applications, a hard fuser roller is used that comprises a cylindrical body of metal coated with polytetrafluoroethylene (PTFE) commercially available under the trademark Teflon®. The metallic fuser roller is durable and highly immune to abrasion or other damage caused by continuous contact with the media stripping claw. For multi-color printing applications, on the other hand, a fuser roller is covered with an outer elastic layer of fluorine-coated silicone rubber, such as a several tens micron-thick tubular coating of perfluoroalkoxy, or with a coating of oil or fluorine resin deposited on the silicone rubber layer. The rubber-covered roller allows for good reproduction of color, while relatively vulnerable to damage as the outer elastic layer readily abrades due to continuous contact with the media stripping claw, resulting in undesired streaks or other imperfections in a resulting image.
A practical approach to prevent damage to the fuser member and concomitant image defects is to use a non-contact media stripper mechanism that can strip a recording medium without touching the fuser member. One example of such non-contact media stripper is an elongated mechanical assembly, such as a thin-edged stripping plate or a plurality of interspaced stripping fingers arranged in line, which extends parallel to a length of the fuser member with a spacing of approximately 0.2 mm to approximately 1.0 mm left between the stripping member and the fuser member. Another example is a self-stripping system in which a recording medium separates from an elastic curved surface of a fixing member due to its own stiffness and the elasticity of the fixing member.
Although effective for their intended purposes, the non-contact stripping methods depicted above would not work properly, where the recording medium exhibits a greater than usual tendency to adhere to the fixing member and escape the stripping member to enter a space between the fixing member and the adjoining guide structure, such as in the case of processing thin paper, a densely printed page with a narrow margin on the leading edge, or a solid or photographic print. Such failure in stripping the recording medium from the fixing member eventually cause the recording medium to wrap around the fuser member, or otherwise to unduly interfere with the media stripper, resulting in paper jam and other concomitant conveyance failure in the fixing nip.
To promote better functioning of a non-contact media stripper, several methods have been proposed which employ a pneumatic nozzle in combination with a mechanical stripping member. Upon activation, the pneumatic nozzle directs compressed air to an interface between a fuser member and a printed face of a recording medium, so as to pneumatically force the recording medium to separate from the fuser member at the exit of a fixing nip. In a sophisticated, power-efficient configuration, such a pneumatic nozzle is activated only temporarily where the leading edge of a recording medium exits the fixing nip, which reduces the load and size of an air compressor required to operate the pneumatic stripper mechanism.